Blender 2.8 GUI lag when rendering (F12 & viewport render preview)
As a workaround, I am now using Blender 2.80 on a Kubuntu 18.04.3 system. The interface, while not perfect, works much smoother than on Windows (while rendering viewport view).

Since the nVidia driver doesn't get loaded on Kubuntu if the Intel GPU is selected (sudo prime-select intel), I am forced to use the nVidia GPU as primary, and so Blender uses it for both rendering of interface and rendering in Cycles. But this is still so much smoother than on Windows 10. The GUI elements react quickly to mouse interactions.